Chemically Stable Visible Light Photoemission Electron Source

ABSTRACT

A method of producing electrons via photoemission comprising providing diamond, treating a surface of the diamond by exposing it to atomic hydrogen inside an ultrahigh vacuum chamber, illuminating the surface with photons, and extracting the photoemitted electons. A chemically stable visible light photoemission electron source comprising a diamond film having a surface terminated with hydrogen and a light source.

This application claims priority to and the benefits of U.S. PatentApplication No. 61/864,854 filed on Aug. 12, 2013, and U.S. patentapplication Ser. No. 14/306,122 filed on Jun. 16, 2014, the entirety ofeach is herein incorporated by reference.

BACKGROUND

Electrons bound inside a solid can be promoted to vacuum by one or acombination of three major techniques: thermionic emission, fieldemission, or photoemission. Each method has advantages anddisadvantages.

The most common method is thermionic emission, a process that requiresheating the cathode (electron-emitting) material to high temperatures.The high temperature creates a fundamental limit on the lifetime of thecathode.

Field emission can occur at low temperatures, but the need to create avery high electric field at the emission site and issues associated withthe high electric field limit the application of field emission sources.

The third major method of producing vacuum electrons is photoemission.Advantages of photoemission include the ability to control the time andlocation of the emission by changing the time and location of theincident light. For example, photoemission is often used to produceelectron pulses too short to be produced by other means, typically lessthan about 1 nS. At normal intensities the energy of the illuminatingphotons hv must exceed the minimum required energy E_(min) to removeelectrons from the emitting material. For a metallic surface, E_(min) isthe work function of the surface. The range of energies produced ishv-E_(min). Normally it is necessary to make hv significantly largerthan E_(min), since the value of E_(min) varies with location across thesurface and with the chemical condition of the surface.

Chemically stable metals typically have E_(min) near or above 4.5 eV, sohv must be greater than 4.5 eV, which is in the ultraviolet. Typicallyultraviolet light sources are more expensive and more difficult to workwith than visible sources. The energy E_(min) can be reduced by coatingsome surfaces with special materials such as alkali metals and theiroxides or salts (such as Cs, CsO, or CsI). However, these materials arechemically reactive, making then difficult to work with and limiting theuseful lifetime of photo-cathodes that use such coatings.

Many devices require electron beams that can be collimated or focused toa small spot. These qualities are limited by the size of the electronsource and the kinetic energy transverse to the beam direction producedby the electron source. The product of the source size and transverseenergy is proportional to the emittance. Equal emittance, and thus equalbeam quality, can be achieved by trading off emission area andtransverse energy.

Diamond surfaces terminated with hydrogen have negative electronaffinity (NEA), such that electrons with energies at or above theconduction band minimum can be emitted to vacuum without gainingadditional energy. This property allows efficient electron emission forelectrons in the conduction band when those electrons encounter the NEAsurface. The fraction of electrons that encounter the surface isenhanced or diminished by negative or positive electric field in thediamond between the electron and the surface.

Promoting an electron from the diamond valence band directly to theconduction band requires adding at least slightly more than the band gapenergy (5.5 eV) to the electron, requiring light with wavelength λ<220nm. Light with energy less than 5.5 eV can also promote electrons to theconduction band via less efficient two-step or multi-step processesinvolving energy states in the bandgap caused by crystal latticeimperfections such as impurity atoms or carbon vacancies. Thisabsorption rapidly becomes weaker with longer wavelength, such that theratio of emitted electrons to incident photons (the quantum efficiencyor QE) becomes lower for longer wavelengths. Most reported measurementsof photoemission from diamond showed undetectable emission current forlight wavelengths λ>275 nm. One report showed a detectable emissionthreshold near 3.0 eV (413 nm), however no repeatable process wasdescribed to produce the low energy emission.

BRIEF SUMMARY OF THE INVENTION

We disclose an apparatus and method for producing electrons viaphotoemission using relatively low energy (blue-violet) photonsavailable from inexpensive light sources such as laser diodes, withoutapplying chemically reactive materials such as Cs or Ba to the emissionsurface.

In particular we have used a laser diode producing wavelengths near 405nm. Such lasers are physically small, easy to operate, and are availablecommercially at low cost with optical output power up to about 1 Watt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the integrated emission intensity in the emissionspectra of one polycrystalline diamond film as a function of 405 nmlaser power.

FIG. 2 illustrates an energy distribution of the electrons emitted fromdiamond using 405 nm light.

DETAILED DESCRIPTION

Here we disclose an apparatus and method for producing electrons viaphotoemission using relatively low energy (blue-violet) photonsavailable from inexpensive light sources such as laser diodes, withoutapplying chemically reactive materials such as Cs or Ba to the emissionsurface. In particular we have used a laser diode producing wavelengthsnear 405 nm. Such lasers are physically small, easy to operate, and areavailable commercially at low cost with optical output power up to about1 Watt.

This invention concerns an apparatus and method to produce vacuumelectrons.

The vacuum electrons are produced via photoemission, to provide emissionfrom a specific area and time corresponding to the location and time ofthe illumination intensity, and to allow a rapid change in the emissioncurrent and/or the emission location.

The vacuum electrons are produced using readily available light sourceswith high power and low cost, especially sources that might be easilypackaged with the photoemitter and which can be used to produce time- orlocation-modulated intensity.

The method and apparatus use light wavelengths which are transmitted bycommon optical materials such as BK7 glass and some polymers, from achemically stable (not chemically reactive) surface, and with a narrowrange of energy.

This invention produces vacuum electrons from a solid, inert surfacethermally, mechanically, and chemically compatible with micro-fabricatedelectrodes and structures made from metallic, semiconducting, ordielectric materials. Such structures can be used to shape the emittingarea, and/or control the current, energy, and trajectory of the emittedelectrons.

This invention produces vacuum electrons by illuminating from the sideopposite the emitting surface, such that the light source and focusingelements can be placed close to the electron emitting surface withoutblocking or deflecting the emitted electrons.

This invention has many applications, including scanning electronmicroscopes and electron beam patterning machines.

EXAMPLE 1

We have hydrogenated several p-type diamond samples including specimensgrown in our laboratory and a purchased specimen. Specimens that werehydrogenated by ex-situ exposure to hydrogen plasma exhibited emissionthreshold energies below the conduction band minimum, but did notproduce photo-emission using 405 nm light. The diamonds we tested weredoped with boron at levels near 1-5×10¹⁶cm⁻³ and were conductive enoughto prevent significant changes in surface potential during the emissionmeasurements. The diamond we used was grown by plasma CVD on siliconsubstrates and on diamond substrates. Similar diamond may be grown onother substrates. We removed one of the diamond films from its substrateand bonded it over a hole in a separate substrate (such that a portionof the film was suspended).

The diamond surface is treated by exposing it to atomic hydrogen insidean ultrahigh vacuum (UHV) chamber. We produced atomic hydrogen bypassing molecular hydrogen through a commercial thermal hydrogen cracker(in our case Omicron EFM-H) consisting of a tungsten capillary tubeheated to ˜1800° C. with an electron beam. Operating this type ofhydrogen cracker requires the chamber pressure to be less than 10⁻⁴torr. Treating the diamond with lower flow rates of atomic hydrogen isalso effective. The cracker is operated in a stainless steel UHV chamberevacuated with a turbo-molecular pump. The molecular hydrogen is firstpurified by passing it through a commercial Pd diffusion purifier. Theline connecting the purifier to the cracker is fitted with a tee leadingto a second turbo pump, such that most of the purified hydrogen ispumped away before passing through the cracker. This bypass constantlyflushes the gas line such that impurities introduced by the stainlesssteel gas line do not accumulate. Before the diamond is treated, thechamber walls are cleaned by heating while operating the atomic hydrogensource for several hours, until the partial pressures of water andhydrocarbon vapors fall below 10⁻⁸ torr. After the cleaning process isover and the chamber has cooled, gases other than hydrogen have partialpressures well below 1×10⁻¹⁰ torr.

The required dose of atomic hydrogen depends on the initial condition ofthe diamond surface; cleaner diamond surfaces are activated morequickly. Typical surfaces may require hydrogen does similar to ˜10⁶monolayers or 3 hours at 10⁻⁴ torr. The dose may be reduced by heatingthe diamond in the vacuum prior to hydrogen exposure or during hydrogenexposure. After exposing the diamond surface to sufficient dose ofatomic hydrogen, photoemission occurs upon illuminating the surface withlight having wavelength similar to or less than 405 nm. We have observedemission threshold energies as low as 2.6 eV (476 nm). Progress may betested by repeatedly dosing with atomic hydrogen and testing theemissison, or by testing the emission while exposing to atomic hydrogen.

To extract the photoemitted electrons, we put the sample at negativepotential with respect to the grounded chamber, thus creating a positiveelectric field in front of the diamond surface. We use an electric fieldof about 10⁴ V/m, produced with a negative potential of −100-200V,limited by our equipment. The emission current we measure increases withthe voltage difference between the diamond and collecting electrode,suggesting that a substantially larger electric field would producehigher emission current.

The integrated emission intensity in the emission spectra of onepolycrystalline diamond film is plotted in FIG. 1 as a function of 405nm laser power. The plot shows a linear behavior, indicating thatadditional laser power would produce proportionally more current.

An energy distribution of the electrons emitted from diamond using 405nm light is plotted in FIG. 2. The full width at half maximum (FWHM) isabout 0.5 eV. In some cases the energy distributions were as little as0.2 eV.

We have verified that shorter wavelength light will also produceelectrons; specifically we have tested 375, 365 and 256 nm light. Thequantum efficiency increases substantially at shorter wavelengths. Themaximum energy of the emitted electrons and the width of the energydistribution also increases when shorter wavelengths are used.

The quantum efficiency (QE) of the source (emitted electrons perincident photon) was approximately 10⁻⁷ under un-optimized conditionswith a 45° incidence angle. The QE varied with H treatment, from placeto place on a given sample, and from sample to sample, indicating the QEcan be improved by optimizing the sample surface. The QE was highestimmediately after H exposure and declined with time after hydrogenexposure, but was typically very stable under UHV conditions.

EXAMPLE 2

One sample was treated with atomic hydrogen, then removed from thechamber and stored a box purged with nitrogen for several weeks. Thesample began to emit immediately after re-inserting into the UHV chamberand baking at 250° C.

Our invention involves diamond, which is chemically inert. Furthermore,the hydrogenated surface is stable in air. The surface can be stored forlong periods.

Still furthermore, diamond has exceptionally high thermal conductivity,such that high light intensities can be applied to the surface.

The hydrogenated diamond surface and the emission temperature iscompatible with structures such as electrodes, which can be applied ormicro-fabricated onto the diamond. Such structures can be used tocontrol the emitting area(s), current, energy, and trajectory of theemitted electrons. For example, an image could be generated in this wayand changed over time.

Our invention involves visible light with wavelength less than about 400nm, which involves several important aspects.

Visible light has high transmission through high quality p-type diamond,such that the illumination can be applied from the side of a diamondfilm opposite the emission side. This allows the light source and/or alens to be placed much closer to the emission surface than could be donefrom the front side and is therefore advantageous over the prior art.

Light emitting diodes and laser diodes producing wavelengths near 405,and 375 nm are easily obtained at low cost.

405 nm light has high transmission through common dielectric materialssuch as glass and some plastics. Thus, these materials can be used aswindows and lenses.

There is a tradeoff between optical power that can be convenientlygenerated using available technology at various wavelengths and electronemission efficiency. Light emitting diodes and lasers providingwavelengths shorter than 405 nm should provide superior quantumefficiency with similar energy spread. At present the shortestwavelength laser diode available with substantial power produces 375 nmlight. Alternatives include doubled or tripled lasers such as a tripledYAG laser which produces about 354 nm. Plasma sources such as Hg arclamps (which produce a strong line at 254 nm) are also effective.

Light emitting diodes and lasers providing wavelengths longer than 405nm (445-450 nm) are available commercially and provide higher power atlower cost than the shorter wavelength sources.

In some applications it may be advantageous to illuminate the diamondphotocathode from the side opposite to the emission side. This can bereadily accomplished since diamond is largely transparent to thesub-bandgap light. The diamond should have the shape of a thin film orplate, optically smooth on both sides, and should be mounted in a waythat leaves both sides of the plate exposed. It is possible to growpoly-crystal diamond over an optically transparent substrate, such thatthe diamond need not be mounted.

Some applications may require emission from a small area or highresolution image. This can be readily accomplished by focusing the lightonto the emission surface of the diamond from the back side. A lensplaced on the emission side of the diamond would typically require along focal length lens to avoid interfering with the electrontrajectory. Positioning the lens immediately behind the diamond plateallows a much shorter focal length lens and axial alignment. Arearesolution of approximately 1 micron can be produced in this way. Sincepassing the light through thicker diamond films (especiallypoly-crystalline films) will tend to diffuse and de-focus the light,obtaining the smallest possible focal spot can involve a thin film ofsingle crystal diamond.

Many modifications and variations of the present invention are possiblein light of the above teachings. It is therefore to be understood thatthe claimed invention may be practiced otherwise than as specificallydescribed. Any reference to claim elements in the singular, e.g., usingthe articles “a,” “an,” “the,” or “said” is not construed as limitingthe element to the singular.

What we claim is:
 1. A method of producing electrons via photoemissioncomprising: providing diamond; treating a surface of the diamond byexposing it to hydrogen inside an ultrahigh vacuum chamber; andilluminating the surface with photons from light having a wavelengthsimilar to 405 nm and thereby photoemitting electrons.
 2. The method ofproducing electrons via photoemission of claim 1 wherein the diamondsubstrate is doped p-type with boron and results in the diamond beingconductive.
 3. The method of producing electrons via photoemission ofclaim 1 wherein the treating a surface of the diamond by exposing it tohydrogen inside an ultrahigh vacuum chamber comprises utilizing atungsten tube to cause low energy photoemission.
 4. The method ofproducing electrons via photoemission of claim 1 wherein the treating asurface of the diamond by exposing it to hydrogen inside an ultrahighvacuum chamber comprises utilizing a tungsten tube heated to about 1800°C.
 5. The method of producing electrons via photoemission of claim 1wherein the treating a surface of the diamond by exposing it to hydrogeninside an ultrahigh vacuum chamber comprises utilizing a tungsten tubeheated to about 1800° C. with hydrogen passing through it.
 6. The methodof producing electrons via photoemission of claim 1 wherein the treatinga surface of the diamond by exposing it to hydrogen inside an ultrahighvacuum chamber comprises utilizing a tungsten tube wherein the tungstentube is oriented with the tube axis aimed at the diamond surface and ashort distance from the diamond surface to cause low energyphotoemission.
 7. The method of producing electrons via photoemission ofclaim 1 wherein the diamond does not comprise grain boundaries.
 8. Themethod of producing electrons via photoemission of claim 1 wherein thehydrogen exposure is performed inside the same vacuum chamber asphotoemission, and can be performed repeatedly before, during, or afterthe photoemission.
 9. A chemically stable visible light photoemissionelectron source comprising: a diamond film having a surface terminatedwith hydrogen; and a light source having a wavelength of from about 475nm to about 250 nm; wherein electrons are generated at the surface ofthe diamond and photoemitted.
 10. The chemically stable visible lightphotoemission electron source of claim 9 wherein the diamond film has atop surface and a bottom surface, wherein the top surface is exposed tohydrogen and wherein the light source is located near, or the light isdirected to, the bottom surface such that the illumination travelsthrough the diamond.
 11. The chemically stable visible lightphotoemission electron source of claim 9 wherein the electrons movedirectly from the surface into a vacuum without first being excited intothe conduction band.
 12. The chemically stable visible lightphotoemission electron source of claim 9 wherein the light source has awavelength of about 405 nm or less.
 13. The chemically stable visiblelight photoemission electron source of claim 9 wherein the light sourcehas a wavelength of 475 nm or less or is one selected from the groupconsisting of 450 nm, 405 nm, 375 nm and 354 nm.
 14. The chemicallystable visible light photoemission electron source of claim 9 whereinthe temperature of the diamond film is increased in order to improve thequantum efficiency of emission.
 15. The chemically stable visible lightphotoemission electron source of claim 9 wherein the temperature of thediamond film is changed in order to change the lowest energy of emittedelectrons according to the change in the diamond band gap energy. 16.The chemically stable visible light photoemission electron source ofclaim 9 wherein the diamond is exposed to hydrogen during emission inorder to improve the quantum efficiency of emission.
 17. The chemicallystable visible light photoemission electron source of claim 9 whereinthe diamond is exposed to ultraviolet light created by hydrogen insteadof or in addition to other light sources in order to cause emission.